In the case of the filtration of liquid media or liquids, for example in the case of the filtration of beverages, e.g. beer, as a rule, filters, e.g. tank filters also, with filter cartridges are used, the filters making necessary the addition of a filter aid, e.g. diatomite, in order to obtain the desired filtration result.
The filter aid, which is mixed with the unfiltered product, is deposited on a filter element that separates an unfiltered space from a filtered space, for example on the outside of the filter cartridge, and there forms a filter layer, which accommodates the substances to be filtered out of the unfiltered product, such as sludge.
For an optimum filtration result it is necessary to add a certain quantity of filter aid to the medium or product to be filtered per unit (quantity, weight or volume). In this case it must be kept in mind that through the geometric dimension of the respective filter, for example through the geometric dimension of the tank filter, the filter cartridge, the space between each of the filter cartridges, etc., only a certain maximum quantity of filter aid can be accommodated in the relevant filter. Once this maximum quantity has been reached, the relevant filter has to be cleaned and/or back-flushed which causes interruption to the normal production.
In order, on the one hand, to obtain the optimum filtration result and, on the other hand, to keep the times between two cleaning and back-flushing operations as great as possible, attempts are made to meter the addition or the admixing of the filter aid to the medium or unfiltered product to be filtered in as precise a manner as possible. Up to now, in particular, this has not been possible in an automated manner or has only been possible with very unsatisfactory results.
Normally the filter aid is supplied to the unfiltered product as a suspension, which contains the filter aid in a liquid, for example in water or in the medium to be filtered. The production of the suspension is then effected, for example, by means of an agitator, by means of which the filter aid is mixed with the liquid and is slurried to form a suspension.
In the case of known methods for creating the suspension, however, continuous manual interventions are necessary on the part of the operating personnel and this, among other things, leads to the composition of the suspension, i.e. the proportion of the filter aid in the suspension, not being known with the necessary amount of precision. The result of this is that a simple quantity-controlled and/or volume-controlled adding of the suspension to the medium or unfiltered product to be filtered does not result in the filter aid being metered with a sufficient degree of precision.
To this must be added that the metering pumps normally used for adding the suspension have considerable inaccuracies, such that, when seen overall, automated metering of the filter aid has not been possible up to now with a satisfactory degree of precision.
In addition, it is known to investigate the filtrate flow, i.e. the liquid medium after the filtration, for the presence of slurry, for example also for the presence of filter aid, for example by using a scattered light turbidity measurement or by an absorption measurement. Using these known methods it is possible in a rough manner to determine or indicate changes in concentration, among other things, of the filter aid in the filtrate, in no way, however, is it possible to determine and/or indicate the actual content of the filter aid in the filtrate. Using the known methods neither is it possible, in particular, to determine the quantity of the filter aid supplied to the unfiltered product and consequently to the respective filter, nor to control, regulate or define it in an automated manner. In this case, in particular, the determining of the total quantity of the filter aid supplied to a filter is important as each filter can only accommodate a certain quantity of filter aids, and as exceeding the quantity, so-called overloading the filter, as a rule results in damaging the filter.